1. Field
Exemplary embodiments relate to a method of fabricating a nitride substrate. More particularly, exemplary embodiments relate to a method of fabricating a nitride substrate having high quality and low density of defects such as dislocations.
2. Discussion of the Background
Among III-V based compound semiconductors, nitride semiconductors (i.e., aluminum-based nitride, gallium-based nitride, and indium-based nitride semiconductors) have excellent electromagnetic characteristics. Thus, nitride semiconductors have been recently been utilized in light emitting diodes, photo-detectors, and high-speed electronic devices. In particular, gallium nitride (GaN) is commonly used to manufacture semiconductor devices because gallium nitride has an energy band-gap of 3.4 eV and has direct transition type characteristics.
In fabrication of a gallium nitride semiconductor, a homogeneous substrate along with the gallium nitride semiconductor may be used as a growth substrate. However, gallium nitride has a melting point of over 2,000° C. and a very high nitrification vapor pressure. Thus, it is difficult to fabricate an ingot of gallium nitride and a gallium nitride substrate. To make the fabrication of gallium nitride easier, a heterogeneous substrate (i.e., sapphire (Al2O3) substrate, a silicon carbide (SiC) substrate, and a silicon (Si) substrate) is used as a growth substrate instead of a homogeneous substrate. However, the gallium nitride semiconductor fabricated using the heterogeneous substrate typically has a high defect density due to a difference in the lattice parameter and the coefficient of thermal expansion between the heterogeneous substrate and the gallium nitride semiconductor. Such a high defect density can cause deterioration in the operational characteristics of the gallium nitride semiconductor.
A gallium nitride substrate is generally fabricated by growing a single nitride crystal layer to hundreds of micrometers on a heterogeneous substrate, such as a sapphire substrate or a silicon carbide substrate, through metal organic chemical vapor deposition (MOCVD) or hydride vapor phase epitaxy (HVPE), followed by separating the heterogeneous substrate from the single crystal layer through laser lift-off (LLO).
However, MOCVD allows growth of a gallium nitride epitaxial layer at a rate of several hundred micrometers per hour at most. Thus, it is very difficult to grow a gallium nitride substrate having a thickness of hundreds of micrometers. Also, the effectiveness of MOCVD is low due to high costs of metal organic sources and equipment operation. In contrast, HVPE allows growth of a gallium nitride epitaxial layer at a rate of 100 μm/hour or more. Thus, it is possible to fabricate a gallium nitride substrate having a thickness of hundreds of micrometers within a relatively short period of time. In addition, since HVPE employs gallium and ammonia gas as Ga and N sources, HVPE is more economical than MOCVD.
As briefly described above, sapphire used for growth of the gallium nitride epitaxial layer causes generation of stress and strain from an interface between a sapphire substrate and the gallium nitride epitaxial layer due to a significant difference in the lattice parameter (about 16%) and the coefficient of thermal expansion (about 35%) between the sapphire substrate and the gallium nitride epitaxial layer. Such stress and strain may cause lattice defects, bowing and cracking of the gallium nitride epitaxial layer, and deterioration in the quality of the manufactured gallium nitride substrate. In particular, when the gallium nitride epitaxial layer is grown using HVPE, the grown gallium nitride epitaxial layer typically has a relatively high defect density and may suffers from frequent bowing and cracking due to a very high growth rate. In addition, when the gallium nitride epitaxial layer is grown to a thickness of 10 μm or more on the sapphire substrate, thicker gallium nitride epitaxial layer may reduce a radius of curvature, thereby causing a higher degree of bowing. As a result, stress is further increased, thereby causing a higher density of cracks. Furthermore, when temperature is lowered after growth of the gallium nitride epitaxial layer, local cracking at a high stress region of the epitaxial layer can be propagated to the entirety of the epitaxial layer, and a gallium nitride substrate made from the gallium nitride epitaxial layer having cracks cannot be used as a growth substrate for a gallium nitride semiconductor.
Moreover, when a laser is employed to separate the heterogeneous substrate, such as a sapphire substrate from the gallium nitride epitaxial layer, the substrate is gradually separated from a partial portion of the interface between the substrate the gallium nitride epitaxial layer to the entirety thereof since a laser beam has a much smaller cross-sectional area than the overall area of the interface. As a result, stress and strain are first released at a portion of the interface, thereby causing cracking of the gallium nitride epitaxial layer or fracture of the entire substrate.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the inventive concept, and, therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.